images: Implement collect_images() to minimaze code lines

Pack repeated actions in one. Also, fixed the "return -1"
from the middle of restore_task_with_children(),
while cleanup is need.

v2: goto err in case of error instead of return -1.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>